The only way I know to get curls (more like waves) on synthetic hair is using string and hot water. I am guessing this works with all synthetic hair, but I'm not sure as when I used to use synthetic, I had no idea there were so many kinds.
Basically after braiding to the end...and sealing however you choose. You wrap string around several braids held straight. You first wrap the base as you would to make a ponytail to fasten the string and then you wrap the length of the braid loosely...the way one of Queen_T's curlformer set spiral might wrap around the hair
Then holding the string taut, you push the braid to the base so they form a tight coil. You wrap the exposed part of the string around the top of the coil to hold it in place. Then you dip it in hot water while in that coil position and immediately squeeze out water with a towel. It can be a dangerous endeavor as it's easy for the water to drip on your scalp, neck or face. Then let it cool down or dry. When you undo, you'll have wavy braids.
I don't know if my explanation makes sense. Maybe someone has a youtube video on how to do this.